The present invention relates to a process of producing an .alpha.-olefin and, more particularly, to a process of producing an .alpha.-olefin containing a linear .alpha.-olefin in high purity.
An .alpha.-olefin is an olefin oligomer of carbon number 4 to 18 which has a straight-chain (unbranched) molecular structure in which there is a double bond at the .alpha. position. An .alpha.-olefin, especially the one containing a linear .alpha.-olefin in high purity, finds use as a monomer to modify polyolefins or as a raw material for synthetic lubricants, plasticizers, and surface active agents. Especially, those of carbon number 6 to 10 are in general use. For example, an .alpha.-olefin of carbon number 6 to 8 is used mainly as a comonomer of polyolefins. It is required to be linear and highly pure, and it enjoys a large demand. Also, there is in an increasing demand for an .alpha.-olefin of carbon number 10 or so as a raw material for synthetic lubricants.
An .alpha.-olefin is produced conventionally by polymerization of ethylene by the use of a Ziegler catalyst. There are two known processes: one gives rise to an .alpha.-olefin whose molecular weight distribution obeys the Schultz-Flory distribution, and the other gives rise to an .alpha.-olefin whose molecular weight distribution obeys the Poisson distribution. The former process usually employs a titanium-based catalyst or a zirconium-based catalyst to convert ethylene into an .alpha.-olefin oligomer. Its advantage is the ability to give rise to an .alpha.-olefin containing a linear .alpha.-olefin in high purity (with small amounts of branched .alpha.-olefin). On the other hand, it suffers the disadvantage of giving rise to an .alpha.-olefin whose molecular weight distribution is usually broad and obeys the Schultz-Flory distribution. If it is used to produce an .alpha.-olefin of carbon number 10 or below in large amounts which has a comparatively narrow molecular weight distribution, the resulting product contains .alpha.-olefins of carbon number 4 to 6 (such as butene) in an extremely large amount. In other words, it is not suitable for the efficient production of .alpha.-olefins of carbon number 10 or below, which find use as a comonomer of polyolefins or a raw material of synthetic lubricants.
On the other hand, there is a process of oligomerizing ethylene into an .alpha.-olefin by the use of trialkyl aluminum (such as triethyl aluminum and tributyl aluminum). It offers the advantage of giving rise to an .alpha.-olefin whose molecular weight distribution obeys the Poisson distribution and selectively giving rise to an .alpha.-olefin of carbon number 10 or below. In addition, there has been proposed a process of recycling the resulting butene, thereby reducing the amount of butene produced. (See Japanese Patent Laid-open No. 48630/1991.)
This process consists of performing the growth of ethylene with trialkyl aluminum at a comparatively low temperature under high pressure, thereby forming a higher trialkyl aluminum, replacing a large part of the higher trialkyl aluminum with ethylene at a comparatively high temperature under low pressure, thereby forming an .alpha.-olefin and regenerating triethyl aluminum, replacing the remaining higher trialkyl aluminum with butene contained in the thus formed .alpha.-olefin, thereby forming an .alpha.-olefin and tributyl aluminum, and performing the growth of ethylene with tributyl aluminum. Thus, butene is consumed for the formation of tributyl aluminum, and the final product is an .alpha.-olefin of carbon number 6 or above. The disadvantage of recycling butene is that the resulting .alpha.-olefin contains branched .alpha.-olefins, with the purity of linear .alpha.-olefins being low.
The present inventors found that the low purity of .alpha.-olefins produced by the above-mentioned process arises from the displacement of higher trialkyl aluminum with butene (which is carried out to recycle butene) and the separation by distillation of the product formed by displacement. They investigated the steps of utilizing the butene in a different manner than mentioned above, and the result of the investigation led to the present invention.